Conventionally, in electret condensers, which are usually applied to elements for condenser microphones and the like, a structure is employed which includes an electret film as dielectrics having a permanent electric polarization and an air gap (cavity) layer between a fixed electrode and a movable electrode which compose a parallel-plate condenser.
In such electret condensers, the thickness of the air gap layer has a direct relationship with a capacitance value of the condenser and involves significant influence on performance of the microphone and the like. Specifically, as the air gap layer is thin, the sensitivity of the microphone or the like increases. In contrast, when variation in thickness of the air gap layer in the manufacturing process is large, variation in sensitivity increases in the microphone and the like. Accordingly, the air gap layer provided in the electret condenser is desired to be thin and has less variation in thickness in the manufacturing process.
In recent years, in order to reduce the thickness of the air gap layer and variation in the thickness thereof in the manufacturing process, a structure of an air gap layer and a method for manufacturing it which utilize a microfabrication technology have been proposed. Specifically, for example, a technique has been proposed in which part of a Si (silicon) substrate is removed by wet etching using potassium hydroxide to form a recess (see Patent Document 1).
Patent Document 1: Japanese Patent Application Laid Open Publication No. 2002-345088A